1. Field of the Invention
The present invention relates to a tin borophosphate glass and to a sealing material using the glass.
2. Description of the Related Art
Materials using glasses have been developed as sealing materials for glass, ceramics, metals, and other materials.
To obtain a strong bonding, such a sealing glass must be heated to a sufficiently high temperature to wet the surface of a subject to be sealed in a sealing process. The sealing temperature should be, however, decreased as low as possible in the sealing of electronic parts. Sealing materials are mainly used in such applications and contain a lead-borate low-melting glass. Especially, sealing materials in wide use have a sealing temperature of 430.degree. C. to 500.degree. C. and a thermal expansion coefficient of 70.times.10.sup.-7 /.degree. C. to 100.times.10.sup.-7 /.degree. C.
However, demands have been made in recent years to provide lead-free sealing glasses from the viewpoint of environmental issues.
A tin borophosphate glass has been proposed as such a lead-free sealing glass. This type of glass contains, however, large amounts of P.sub.2 O.sub.5 as a main glass forming oxide and shows various disadvantages specific to a phosphate glass and has not yet been used in practice. The disadvantages of phosphate glasses include 1) an increased thermal expansion coefficient, 2) a deteriorated mechanical strength as compared with conventional sealing glasses, 3) a deteriorated weatherability, 4) exuding of a remaining matrix glass when crystallized, and 5) vigorous foaming in a glass forming reaction due to a material phosphoric acid.